Sunday marks the 15th anniversary of the attacks of Sept. 11, and TODAY is remembering the day — and the time since — with some of the people who lived through it.

From students to firefighters, to individuals working at the World Trade Center that day, those who will "never forget" shared stories of strength and adversity, detailing the challenges they've overcome — as well as the their hopes for the future.

Related: Rebuilding the World Trade Center: 15 years after 9/11

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Rebuilding the World Trade Center: 15 years after 9/11

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Rebuilding the World Trade Center: 15 years after 9/11

A group of firefighters walk amid rubble near the base of the destroyed south tower of the World Trade Center in New York on September 11, 2001. In the worst terror attack on the U.S. mainland in modern history, two hijacked planes slammed into the twin towers of the World Trade Center in New York and a third plane hit the Pentagon, across the Potomac river from Washington. REUTERS/Peter Morgan

Family members of the victims of the the attacks on the World Trade Center enter Ground Zero to pay their respects, September 11, 2003 in New York. U.S. President George W. Bush on Thursday commemorated the attacks with subdued events of remembrance, as his government warned of possible bigger terror attacks. REUTERS/Stephen Chernin/POOL SC/GN

People gather at the World Trade center site in New York City, September 11, 2004 on the third anniversary of the attacks on the Twin Towers. This year, it is the parents and grandparents of World Trade Center victims who will read aloud the names of those lost on Sept. 11, 2001.

Family members make their way down a ramp to the site of the former World Trade Center during ceremonies marking the fourth anniversary of the attack of the twin towers in New York September 11, 2005. Families of victims of the collapse of the World Trade Center were allowed down to two pools of water placed on the ground where the buildings once stood. REUTERS/Gary Hershorn GMH/VP

People gather around a reflecting pool at the bottom of the Ground Zero site of the World Trade Center on the fifth year anniversary of the September 11, 2001 attacks in New York, September 11, 2006. REUTERS/Mike Segar (UNITED STATES)

Family members of victims pay their respects at the site of the former twin World Trade Center towers in New York September 11, 2007 on the sixth anniversary of the Sept. 11 attacks. The first steel beams of the new Freedom Tower being constructed on the site are seen in the foreground. REUTERS/Gary Hershorn (UNITED STATES)

Family members of victims pay their respects at the site of the former twin towers on the eighth anniversary of the attacks on the World Trade Center, in New York, September 11, 2009. Families of the victims are gathering at the annual ceremony to remember the attacks that killed more than 2,700 people with the destruction of the World Trade Center on September 11, 2001. REUTERS/Gary Hershorn (UNITED STATES DISASTER ANNIVERSARY)

Police and firefighters surround the reflecting pool at the World Trade Center site during 9/11 remembrance ceremonies in New York, September 11, 2010. REUTERS/Don Emmert/Pool

The North Memorial Pool, with 1 World Trade Center under construction at rear, is pictured during ceremonies marking the 10th anniversary of the 9/11 attacks on the World Trade Center, in New York September 11, 2011. REUTERS/Robert Deutsch/Pool (UNITED STATES - Tags: ANNIVERSARY DISASTER)

World Trade Center 1 looms over the north reflecting pool at the 9/11 Memorial during ceremonies marking the 12th anniversary of the 9/11 attacks on the World Trade Center in New York September 11, 2013. REUTERS/Allan Tannenbaum/Pool (UNITED STATES - Tags: DISASTER ANNIVERSARY)

The One World Trade Center building on the early morning of the 14th Anniversary of the terrorist attacks, on September 11, 2015 in New York.. AFP PHOTO/KENA BETANCUR / AFP / KENA BETANCUR (Photo credit should read KENA BETANCUR/AFP/Getty Images)

The World Trade Center. New York.

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"My message to people is: 'We will all be touched by adversity, or wounded at some point in our lives, but we can refuse to be held by it.' And I think that's why I survived," explained Lauren Manning, who was in the North Tower at 8:46 a.m. when the first plane hit.

"Whatever it is, you just can't give up. Because there's always hope for a new beginning, if we work for it," explained Helaina Hovitz, who was in middle school a few blocks away from the World Trade Center.